Ion source with external RF antenna

ABSTRACT

A radio frequency (RF) driven plasma ion source has an external RF antenna, i.e. the RF antenna is positioned outside the plasma generating chamber rather than inside. The RF antenna is typically formed of a small diameter metal tube coated with an insulator. A flange is used to mount the external RF antenna to the ion source. The RF antenna tubing is wound around the flange to form a coil. The flange is formed of a material, e.g. quartz, that is essentially transparent to the RF waves. The flange is attached to and forms a part of the plasma source chamber so that the RF waves emitted by the RF antenna enter into the inside of the plasma chamber and ionize a gas contained therein. The plasma ion source is typically a multi-cusp ion source.

RELATED APPLICATIONS

[0001] This application claims priority of Provisional Application Ser.No. 60/382,674 filed May 22, 2002, which is herein incorporated byreference.

GOVERNMENT RIGHTS

[0002] The United States Government has the rights in this inventionpursuant to Contract No.DE-AC03-76SF00098 between the United StatesDepartment of Energy and the University of California.

BACKGROUND OF THE INVENTION

[0003] The invention relates to radio frequency (RF) driven plasma ionsources, and more particularly to the RF antenna and the plasma chamber.

[0004] A plasma ion source is a plasma generator from which beams ofions can be extracted. Multi-cusp ion sources have an arrangement ofmagnets that form magnetic cusp fields to contain the plasma in theplasma chamber. Plasma can be generated in a plasma ion source by DCdischarge or RF induction discharge. An ion plasma is produced from agas which is introduced into the chamber. The ion source also includesan extraction electrode system at its outlet to electrostaticallycontrol the passage of ions from the plasma out of the plasma chamber.

[0005] Unlike the filament DC discharge where eroded filament materialcan contaminate the chamber, RF discharges generally have a longerlifetime and cleaner operation. In a RF driven source, an induction coilor antenna is placed inside the ion source chamber and used for thedischarge. However, there are still problems with internal RF antennasfor plasma ion source applications.

[0006] The earliest RF antennas were made of bare conductors, but weresubject to arcing and contamination. The bare antenna coils were thencovered with sleeving material made of woven glass or quartz fibers orceramic, but these were poor insulators. Glass or porcelain coated metaltubes were subject to differential thermal expansion between the coatingand the conductor, which could lead to chipping and contamination. Glasstubes form good insulators for RF antennas, but in a design having aglass tube containing a wire or internal surface coating of a conductor,coolant flowing through the glass tube is subject to leakage uponbreakage of the glass tube, thereby contaminating the entire apparatusin which the antenna is mounted with coolant. A metal tube disposedwithin a glass or quartz tube is difficult to fabricate and only has afew antenna turns.

[0007] U.S. Pat. Nos. 4,725,449; 5,434,353; 5,587,226; 6,124,834;6,376,978 describe various internal RF antennas for plasma ion sources,and are herein incorporated by reference.

SUMMARY OF THE INVENTION

[0008] Accordingly, it is an object of the invention to provide animproved plasma ion source that eliminates the problems of an internalRF antenna.

[0009] The invention is a radio frequency (RF) driven plasma ion sourcewith an external RF antenna, i.e. the RF antenna is positioned outsidethe plasma generating chamber rather than inside. The RF antenna istypically formed of a small diameter metal tube coated with aninsulator. A flange is used to mount the external RF antenna to the ionsource. The RF antenna tubing is wound around the flange to form a coil.The flange is formed of a material, e.g. quartz, that is essentiallytransparent to the RF waves. The flange is attached to and forms a partof the plasma source chamber so that the RF waves emitted by the RFantenna enter into the inside of the plasma chamber and ionize a gascontained therein. The plasma ion source is typically a multi-cusp ionsource.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In the accompanying drawings:

[0011] FIGS. 1-5 are side cross sectional views of various embodimentsof a plasma ion source with an external RF antenna according to theinvention.

[0012]FIGS. 6A, B are end and side views of a flange for mounting anexternal antenna to a plasma ion source according to the invention.

[0013]FIG. 7 is a graph of the relative amounts of various hydrogen ionspecies obtained with an external antenna source of the invention.

[0014]FIG. 8 is a graph of hydrogen ion current density extracted froman external antenna source and from an internal antenna source, at thesame extraction voltage.

[0015]FIG. 9 is a graph of the electron current density produced by anexternal antenna source.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The principles of plasma ion sources are well known in the art.Conventional multicusp plasma ion sources are illustrated by U.S. Pat.Nos. 4,793,961; 4,447,732; 5,198,677; 6,094,012, which are hereinincorporated by reference.

[0017] A plasma ion source 10, which incorporates an external RF antenna12, is illustrated in FIG. 1. Plasma ion source 10 is preferably amulti-cusp ion source having a plurality of permanent magnets 14arranged with alternating polarity around a source chamber 16, which istypically cylindrical in shape. External antenna 12 is wound aroundflange 18 and electrically connected to a RF power source 20 (whichincludes suitable matching circuits), typically 2 MHz or 13.5 MHz.Flange 18 is made of a material such as quartz that easily transmits theRF waves. Flange 18 is mounted between two plasma chamber body sections22 a, 22 b, typically with O-rings 24 providing a seal. Chamber bodysections 22 a, 22 b are typically made of metal or other material thatdoes not transmit RF waves therethrough. The chamber body sections 22 a,22 b and the flange 18 together define the plasma chamber 16 therein.Gas inlet 26 in (or near) one end of chamber 16 allows the gas to beionized to be input into source chamber 16.

[0018] The opposed end of the ion source chamber 16 is closed by anextractor 28 which contain a central aperture 30 through which the ionbeam can pass or be extracted by applying suitable voltages from anassociated extraction power supply 32. Extractor 28 is shown as a simplesingle electrode but may be a more complex system, e.g. formed of aplasma electrode and an extraction electrode, as is known in the art.Extractor 28 is also shown with a single extraction aperture 30 but maycontain a plurality of apertures for multiple beamlet extraction.

[0019] In operation, the RF driven plasma ion source 10 produces ions insource chamber 16 by inductively coupling RF power from external RFantenna 12 through flange 18 into the gas in chamber 16. The ions areextracted along beam axis 34 through extractor 28. The ions can bepositive or negative; electrons can also be extracted.

[0020] FIGS. 2-5 show variations of the plasma ion source shown inFIG. 1. Common elements are the same and are not described again or evenshown again. Only the differences or additional elements are furtherdescribed.

[0021] Plasma ion source 40, shown in FIG. 2, is similar to plasma ionsource 10 of FIG. 1, except that flange 18 with external antenna 12 ismounted to one end of a single plasma chamber body section 22 instead ofbetween two body sections 22 a, 22 b. The chamber body section 22 andthe flange 18 together define the plasma chamber 16 therein. Theextractor 28 is mounted directly to the flange 18 in place of the secondbody section so that flange 18 is mounted between body section 22 andextractor 30.

[0022] Plasma ion source 42, shown in FIG. 3, is similar to plasma ionsource 40 of FIG. 2, with flange 18 with external antenna 12 mounted tothe end of a single plasma chamber body section 22. However, ion source42 is much more compact than ion source 40 since the chamber bodysection 22 is much shorter, i.e. chamber 16 is much shorter. In FIG. 2,the length of chamber body section 22 is much longer than the length offlange 12 while in FIG. 3 it is much shorter. Such a short ion source isnot easy to achieve with an internal antenna.

[0023] Plasma ion source 44, shown in FIG. 4, is similar to plasma ionsource 42 of FIG. 3. A permanent magnet filter 46 formed of spacedmagnets 48 is installed in the source chamber 16 of plasma ion source44, adjacent to the extractor 28 (in front of aperture 30). Magneticfilter 46 reduces the energy spread of the extracted ions and enhancesextraction of atomic ions.

[0024] Plasma ion source 50, shown in FIG. 5, is similar to plasma ionsource 42 of FIG. 3, but is designed for negative ion production. Anexternal antenna arrangement is ideal for surface conversion negativeion production. A negative ion converter 52 is placed in the chamber 16.Antenna 12 is located between the converter 52 and aperture 30 ofextractor 28. A dense plasma can be produced in front of the convertersurface. The thickness of the plasma layer can be optimized to reducethe negative ion loss due to stripping.

[0025]FIGS. 6A, B illustrate the structure of a flange 18 of FIGS. 1-5for housing and mounting an external antenna to a plasma ion source.Flange 18 is formed of an open inner cylinder 60 having a diameter D1and a pair of annular end pieces 62 attached to the ends of cylinder 62and extending outward (from inner diameter D1) to a greater outerdiameter D2. Spaced around the outer perimeter of the annular pieces 62are a plurality of support pins 64 extending between the pieces 62 tohelp maintain structural integrity. The inner cylinder 60 and extendingend pieces 62 define a channel 66 in which an RF antenna coil can bewound. The channel 66 has a length T1 and the flange has a total lengthT2.

[0026] The antenna is typically made of small diameter copper tubing (orother metal). A layer of Teflon or other insulator is used on the tubingfor electrical insulation between turns. Coolant can be flowed throughthe coil tubing. If cooling is not needed, insulated wires can be usedfor the antenna coils. Many turns can be included, depending on thelength T1 of the channel and the diameter of the tubing. Multilayeredwindings can also be used. Additional coils can be added over theantenna coils for other functions, such as applying a magnetic field.

[0027]FIG. 7 is a graph of the relative amounts of various hydrogen ionspecies obtained with the source of FIG. 3. More than 75% of the atomichydrogen ion H⁺ was obtained with an RF power of 1 kW. The currentdensity is about 50 mA/cm² at 1 kW of RF input power. The source hasbeen operated with RF input power higher than 1.75 kW.

[0028]FIG. 8 is a comparison of hydrogen ion current density extractedfrom an external antenna source and from an internal antenna source,showing the extracted beam current density from an external antenna andinternal antenna generated hydrogen plasma operating at the sameextraction voltage. When operating at the same RF input power, the beamcurrent density extracted from the external antenna source is higherthan that of the internal antenna source.

[0029] Simply by changing to negative extraction voltage, electrons canbe extracted from the plasma generator using the same column. FIG. 9shows the electron current density produced by an external antennasource. At an input power of 2500 W, electron current density of 2.5A/cm² is achieved at 2500 V, which is about 25 times larger than ionproduction.

[0030] The ion source of the invention with external antenna enablesoperation of the source with extremely long lifetime. There are severaladvantages to the external antenna. First, the antenna is locatedoutside the source chamber, eliminating a source of contamination, evenif the antenna fails. Any mechanical failure of the antenna can beeasily fixed without opening the source chamber. Second, the number ofturns in the antenna coil can be large (>3). As a result the dischargecan be easily operated in the inductive mode, which is much moreefficient than the capacitive mode. The plasma can be operated at lowsource pressure. The plasma potential is low for the inductive mode.Therefore, sputtering of the metallic chamber wall is minimized. Third,plasma loss to the antenna structure is much reduced, enabling thedesign of compact ion sources. No ion bombardment of the externalantenna occurs, also resulting in longer antenna lifetime.

[0031] RF driven ion sources of the invention with external antenna canbe used in many applications, including H⁻ ion production for highenergy accelerators, H⁺ ion beams for ion beam lithography, D⁺/T⁺ ionbeams for neutron generation, and boron or phosphorus beams for ionimplantation. If electrons are extracted, the source can be used inelectron projection lithography.

[0032] A source with external antenna is easy to scale from sizes assmall as about 1 cm in diameter to about 10 cm in diameter or greater.Therefore, it can be easily adopted as a source for either a single beamor a multibeam system.

[0033] Changes and modifications in the specifically describedembodiments can be carried out without departing from the scope of theinvention which is intended to be limited only by the scope of theappended claims.

1. An external RF antenna assembly for a plasma ion source, comprising:an antenna housing and mounting flange adapted to be attached to andform a part of a plasma ion source chamber, and formed of a materialthrough which RF waves are easily transmitted; an RF antenna coil woundon an outside surface of the flange; so that when the flange is attachedto chamber, the antenna coil is external to the chamber, and RF wavesemitted by the antenna coil are directed into the chamber through theflange.
 2. The RF antenna assembly of claim 1 wherein the flange isformed of quartz.
 3. The RF antenna assembly of claim 1 wherein antennacoil is made-of copper or other conducting tubing.
 4. The RF antennaassembly of claim 1 wherein the flange comprises: an open innercylinder; a pair of annular end pieces attached to the ends of cylinderand extending outward; the inner cylinder and extending end piecesdefining a channel in which the RF antenna coil can be wound.
 5. The RFantenna assembly of claim 4 further comprising a plurality of supportpins spaced around the outer perimeter of the annular end pieces andextending between the end pieces to help maintain structural integrity.6. The RF antenna assembly of claim 4 wherein the cylinder and endpieces are made of quartz.
 7. A plasma ion source comprising: a sourcechamber; an RF antenna mounted external to the chamber; an RF powersource coupled to the RF antenna.
 8. A plasma ion source comprising: asource chamber; an external RF antenna assembly mounted to the chamberand with an RF antenna mounted external to the chamber; an RF powersource coupled to the RF antenna.
 9. The plasma ion source of claim 8wherein the external RF antenna assembly comprises: an antenna housingand mounting flange attached to and forming a part of the sourcechamber, and formed of a material through which RF waves are easilytransmitted; an RF antenna coil wound on an outside surface of theflange; so that the antenna coil is external to the chamber, and RFwaves emitted by the antenna coil are directed into the chamber throughthe flange.
 10. The plasma ion source of claim 9 wherein the flange isformed of quartz.
 11. The plasma ion source of claim 9 wherein antennacoil is made of copper or other conducting tubing.
 12. The plasma ionsource of claim 9 wherein the flange comprises: an open inner cylinder;a pair of annular end pieces attached to the ends of cylinder andextending outward; the inner cylinder and extending end pieces defininga channel in which the RF antenna coil can be wound.
 13. The plasma ionsource of claim 12 further comprising a plurality of support pins spacedaround the outer perimeter of the annular end pieces and extendingbetween the end pieces to help maintain structural integrity.
 14. Theplasma ion source of claim 12 wherein the cylinder and end pieces aremade of quartz.
 15. The plasma ion source of claim 8 wherein the sourcechamber is a multi-cusp ion source chamber having a plurality ofpermanent magnets disposed around the chamber.